ENDLicH.] SEDIMENTARY EOCKS ERUPTIVE ROCKS. 97 



rocks belonging to this group. Flowing water takes advantage of existing 

 cracks and Assures, cuts its way along the most favorable route, and re- 

 moveslargequantitiesofbothkindsof rock. This is the case more partic- 

 ularly when the water contains an appreciable percentage of carbonic 

 acid gas. If horizontally, or nearly so, stratified, the limestone beds will 

 show aqueous erosion commensurate with the greater or lesser density of 

 their structure. Cleavage-planes and cracks produced by undermining 

 result in vertical walls, that not unfrequently are found in limestone- 

 areas. Should the strata be inclined, they will present very much the 

 same features we would find in a calcareous sandstone. Instead of 

 sharp ridges we will then see more gentle slopes, worn away gradually. 

 The crest itself will generally be steep, as will also the slopes exposing 

 the edges of strata. 



Vegetation is a powerful agent of decomposition. Not only does it 

 employ the directly-acting mechanical force, which manifests itself dur- 

 ing the period of growth, but it also acts as a chemical agent. Experi- 

 ments have shown that even the most delicate root-fibres of a ])lant 

 will attack the polished surface of marble, of limestone in general. Thus 

 certain portions — those containing the purest calcium carbonate — will 

 be exposed to more vigorous attacks from vegetation than others con- 

 taining admixtures of clay and silica, for instance. Thus an inequality 

 of the surface is eventually produced, which will be corrected at the 

 cost of additional transportation of material from the original stratum^ 

 or strata. 



Dependent upon the chemical constitution of limestones and upoui 

 their structure is their character as drift. Avalanchial drift can be^ 

 found showing both the angular and the rounded bowlders. River and. 

 other water-worn drift is scarcely characteristic enough to admit of any 

 distinction, if we except the peculiar, sometimes indistinct, grooving; 

 upon the pebbles, which is due either to admixture of harder material 

 or to the action of plant-roots. Marble is but a homogeneous crystal- 

 line limestone, and its behavior both under direct mechanical and chem- 

 ical action is entirely in conlbrmity with its physical character. 



ERUPTIVE ROCKS. 



These can appropriately be separated into four groups : 



(cf.) Massive volcanic eruptions, extending in flows over large areas. 

 (h.) Local massive volcanic eruptions, extending but a short distance,, 

 (c.) Volcanic eruptions, in contradistinction to those termed "mas- 

 sive." 

 [d.) Volcanic dikes ajud dike-systems. 



Massive volcanic eruptions are especially well represented in Colorado^. 

 Owing to the regularity and extent of the flows we can appropriately 

 speak of volcanic strata. Wherever rocks belonging to this group have^ 

 been observed they were found to retain the same general character,, 

 varied, of course, by local changes in their physical constitution. 

 Serious disturbances of the strata are comparatively rare, confining: 

 themselves mostly to local faults. Viewed as a whole, we may say that 

 the rocks of this group resemble sandstones more than any others.. 

 Several physical varieties exist, hard, slowly eroding and resisting at- 

 mospheric influences for along time, comparable toquartzitic sandstone;, 

 softer, of irregular physical structure, including tuffs, are analogous in, 

 behavior to argillaceous and partly calcareous sandstones. Conglom- 

 erates are not wanting. 

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