490 WHITMAN CROSS 



VI. The characters derived by chemical diagenesis, or by contact and 

 pressure metamorphism, serve for distinction of lesser groups. 



VII. The altered rocks are to be placed with their original types. 



The system proposed by Walther, in accordance with the 

 stated rules, is in outline the following: 

 I. Mechanical Rocks. Composed of older rock fragments; divided by 

 form and size of the fragments into 5 subgroups : 



1. Breccias: (a) Unstratified ; (b) Stratified; (c) Dike form. 



2. Conglomerates : (a) Stream deposits ; (b) Delta deposits ; (c) Strand 

 deposits. 



3. Moraines. 



4. Psammites — sands, more or less sorted: (a) Quartzose sandstone; 

 (b) Arkose ; (c) Olivine sands; (d) Iron ore sands, etc. 



5. Pelites — of minute particles: (a) Unstratified; (b) Stratified — fresh- 

 water; {c) Stratified — marine. 



II. Chemical rocks. Precipitates or sublimates. 



1. Calcium carbonate. 



2. Calcium sulphate. 



3. Sodium chloride. 



4. Haloid Salts (" Abraumsalze "). 



5. Silica. 



6. Carbon. 



7. Ores. 



Further divisions are made by occurrence — as Stratified, 

 Unstratified, or in Dikes. 



III. Organic rocks. Formed of the remains of animals or plants. 



1. Limestone: (a) Unstratified — -Reef limestone; (b) Stratified — 

 derived from plants (algae) ; (c) Stratified — derived from animals. 



2. Silica : (a) Diatomaceous earth and land plants ; (b) Diatomaceous 

 earth with marine fossils ; (c) Radiolarian earth. 



IV. Volcanic rocks — Consolidated magmas. 



1. Lavas — Compact rocks: {a) Unstratified — deep-seated rocks; (b) 

 Stratified — Effusive rocks ; (c) Dike rocks. 



2. Tuffs — Magmas consolidated in small fragments: (a) Unstratified, in 

 streams, not sorted ; (b) Unstratified, subaqueous accumulations near 

 vent — water tuffs ; (c) Stratified, sorted according to specific gravity; 

 originally inclined ; traversed by dikes — tuffs about a land volcano; 

 dry tuffs ; (d) Stratified, alternating with marine deposits, without 

 dikes ; tuffs of sedimentation ; (<?) tuff dikes or chimneys. 



In connection with these primary rocks the author mentions, 



