地球科學 1788

《地球理論》

詹姆斯·赫頓

陸地本身並不長存——它被無盡地磨蝕、又重建，循環往復，時間沒有起點，也沒有終點。

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In depth · the introduction

你腳下的土地，曾經是海的底部——而總有一天，它還會再成為海底。

核心想法

赫頓湊近去看堅硬的岩石，發現它竟是由更古老岩石的碎屑——砂、貝殼、卵石——構成的：這些碎屑曾沉落在海底，後來變成了石頭。所以陸地並非長存、並非最初就有。它是被回收再造的：今天的大陸，是用從前那些陸地磨損後的殘骸堆起來的。

而且它一直在繼續。河流把陸地磨成泥與沙；這些沉積物在海床上堆積；地球的內熱把它烘成岩石，再把它頂回成新的山脈；接著雨與河又開始把它磨下去。如此循環往復，一個沒有「關機鍵」的循環——而要讓這循環運轉足夠多次、堆出我們眼前的世界，地球就必須老得無法想像。

那個在岩石裡讀出深時的農夫

詹姆斯·赫頓是蘇格蘭啟蒙運動中人——受過醫師訓練，以務農與化工製造為業，是愛丁堡那個璀璨智識圈子裡亞當·斯密（smith-1776）與詹姆斯·瓦特的朋友。幾十年裡看著泥土從自家田地上被雨水沖走，讓他開始思索：失去的泥土都去了哪兒，新的陸地又從何而來。1785 年，他把自己的理論擺到了愛丁堡皇家學會面前；1788 年，他沿蘇格蘭海岸去尋找證據。在錫卡角，他和朋友們找到了：平鋪的紅砂岩層，橫壓在那截被鋸斷、被掀立起來、古老得多的灰色岩石的邊緣之上——一整片懸崖裡，疊著一個世界的兩輪循環。他的同伴約翰·普萊費爾寫道，望進時間的深淵望得那樣遠，人的心智「彷彿要發起暈來」。

它為何重要

在赫頓之前，歐洲多數人相信地球只有幾千歲，而且很久以前一次成形。赫頓把「深時」交給了世界——讓人意識到：過去浩瀚得幾乎無法想像，看不到一個起點。僅此一念，就重塑了其後的一切。它讓萊爾把地質學變成一門嚴謹的科學（lyell-1830）；它給了查爾斯·達爾文那片演化所需的、海洋般浩瀚的時間，去緩緩施展（darwin-1859）。後來幾乎每一幅關於地球歷史的圖景，都立在赫頓鋪下的這塊地板上。

給大陸的一座堆肥堆

想像一座堆肥堆。你扔進枯葉與殘渣；它們腐爛、下沉，化成肥黑的土；你把這土鋪開，種出新的植物，植物死後又回到堆上。沒有什麼是永久的，可也沒有什麼被浪費——它只是在循環。赫頓看見整顆星球都這樣運轉，只不過把枯葉換成了岩石，把「季節」拉長到幾百萬年。山，並不是故事的結局；它們是等著變成堆肥的材料。

之前與之後

赫頓不是第一個注意到山岩裡有海貝的人——尼古拉斯·斯坦諾早在 1669 年就立下了層狀地層的諸原理——但他是第一個把這一切捆成一個運轉著、由熱驅動的循環，並要求一段幾乎無限的過去的人。他自己的文字太過晦澀，這想法險些隨他一同湮沒；它得以存活，是因為約翰·普萊費爾在 1802 年把它清晰重述，也因為查爾斯·萊爾把它建進了那贏得地質學界的三卷書裡（lyell-1830）。從那裡，它徑直奔向達爾文的演化（darwin-1859）；很久以後，放射性為赫頓的「深時」填上了真正的數字——約 45 億年——而板塊構造，終於解釋了他只能用手指點、卻無力解釋的抬升（wegener-1912、hess-1962）。

The original document

Original source text

James Hutton (1726–1797) · “Theory of the Earth; or an Investigation of the Laws observable in the Composition, Dissolution, and Restoration of Land upon the Globe” · Transactions of the Royal Society of Edinburgh, Vol. 1 (1788)

The Earth as a machine

Hutton opens not with rocks but with purpose: the globe is a working machine, contrived so as to remain a fit habitation for living things — and to do so, it must repair itself as fast as it wears away.

When we trace the parts of which this terrestrial system is composed, and when we view the general connection of those several parts, the whole presents a machine of a peculiar construction by which it is adapted to a certain end.

The present land is made of the ruins of a former land

His first observation is the hinge of the whole theory. Look closely at the solid rock of the continents — sandstones, limestones, shales — and you find it built of sand, shells, and the worn debris of older rock: materials that once lay loose on the floor of the sea.

The solid parts of the present land appear, in general, to have been composed of the productions of the sea, and of other materials similar to those now found upon the shores.

Two operations: consolidation and uplift

For loose sea-floor sediment to become dry, solid land, two things are needed, and Hutton attributes both to the Earth's internal heat (his “Plutonist” stance, against the rival “Neptunists” who derived all rock from a universal ocean): first the consolidation of the loose materials into stone, and then the elevation of those consolidated masses from the bottom of the sea to the heights they now occupy. (Annotation of Hutton's argument.)

A circulation in the matter of the globe

Erosion destroys the land; consolidation and uplift build it anew. The two are not opposites but halves of one cycle — a circulation of matter that keeps the world habitable.

We are thus led to see a circulation in the matter of this globe, and a system of beautiful economy in the works of nature.

A succession of worlds

If today's land is built from the wreck of an earlier land, then that earlier land was itself built from one before it — and so on, a succession of former worlds, each recorded where new flat strata rest on the upturned, eroded edges of the old (an angular unconformity, which Hutton read in the field at Siccar Point in 1788).

But if the succession of worlds is established in the system of nature, it is in vain to look for any thing higher in the origin of the earth.

No vestige of a beginning

The cycle has run so many times, over a span so immense, that the rock record shows no first chapter and points to no last. Hutton claims no age in years — his point is precisely that the time is beyond measure.

The result, therefore, of our present enquiry is, that we find no vestige of a beginning,—no prospect of an end.

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Royal Society of Edinburgh · read 1785, published 1788 — expanded into Theory of the Earth, 2 vols., 1795