4 GEOLOGICAL MEMOIRS. 



DIVISIONS AND CLASSES OF ROCKS. 



A. INORGANIC ROCKS.— Rocks, the chief mass of which consists 

 of true mineral substances, and which on heating are neither partly 

 nor entirely volatilized. 



I. Division. Crystalline Rocks. — Rocks of crystalline minerals 

 united together, or combined without any cement. 



1. Class. Simple Crystalline Rocks. — Homogeneous rocks, 

 composed of one mineral substance. 



2. Class. Mixed Crystalline Rocks. — Heterogeneous rocks, 

 consisting of different crystalline mineral species, indistinct, 

 or more or less distinctly visible : (Crypto-, micro-, or macro- 

 crystalline : crypto- and phanero-merous rocks). 



II. Division. Clastic Rocks. — Rocks the mass of which consists of 

 fragments (gravel or grains) of various sizes, rounded or angular, 

 fresh or weathered, which are cemented by an earthy or crystalline 

 matrix ; or also loose (cementless) agglomerations of more or less 

 rounded weathered blocks, gravel, sand, and earthy masses. 



a. Subdivision. Compact fragment-rocks with a cement. 



1. Class. Pseudoclastic rocks. — Large and small, angular, rarely 

 rounded fragments imbedded in a crystalline or slaggy matrix of 

 the same mineral character as the fragments. Partly stratified, 

 partly unstratified. 



2. Class. Hemiclastic Rocks (Tufas). — Matrix dense, porous, soft, 

 friable, more or less earthy, often effervescing with acids, and 

 sometimes resembling clay or chalk (but in no way possessing the 

 physical properties of these substances), and which on lsevigating 

 appear as a mixture of those rocks of which fragments are included 

 in it. Besides the contained fragments, which are often indistinct, 

 this matrix also contains characteristic admixtures of purely cry- 

 stalline minerals. Generally stratified, and often containing or- 

 ganic remains. Greatly resembling the rocks of the previous 

 class, and passing into them. 



3. Class. Holoclastic Rocks. — Matrix more or less compact, earthy, 

 argillaceous, calcareous, areno-argillaceous, areno-calcareous, or 

 marly, with rounded or angular fragments, gravel or grains of 

 rocks having a different mineral character to the matrix. Stratified. 



b. Subdivision. Loose aggregates or detritus. 



1. Class. Boulders, Gravel, Sand. — Determined or named according 

 to their mineral characters. 



2. Class. Soils. — Loose, crumbling aggregates, more or less tena- 

 cious when moist, or crumbling under pressure. 



B. ORGANIC ROCKS Rocks the chief mass of which consists of 



carbon or the product of organic putrefaction, and which, on 



