298 DYNAMICAL GEOLOGY. 



among such projectile material, unless water has aided in the deposition. Projectile 

 work throws out angular fragments broken off from the rocks that adjoin the vent. If the 

 vent ascends through non-volcanic rocks, fragments of these rocks may be distributed 

 along with comminuted igneous material, but they could hardly be a predominant part of 

 the mass. 



2. Rocks. — The rocks of non-volcanic outflows are the same in kinds 

 with those of volcanic origin. The more scoriaceous lavas are usually 

 absent, but vesicular kinds are common. The moisture producing vesicula- 

 tion, and sometimes a general hydrous condition of the rock, may be either 

 that of the deep-seated igneous source, or that of waters taken in on the way 

 to the surface; for the latter method of receiving moisture, — that by molec- 

 ular absorption, if a principle in volcanic phenomena (page 278), will be 

 as much so in non-volcanic. Among the ejections of a system of fissures, 

 those that have come up through sedimentary strata may, or may not, be 

 rendered hydrous, while those intersecting impervious metamorphic rocks 

 are generally anhydrous, with no trace of vesiculation. Owing to such sub- 

 terranean sources of moisture, igneous rocks are sometimes hydrous through- 

 out, and consequently feeble in luster and wanting in durability. In a 

 similar way, the ascending melted rock sometimes gathers in bituminous 

 materials from carbonaceous shales, and puts them into the vesicles. 



Igneous rocks are sometimes divided into those of deep-seated origin 

 related in character to granite, syenyte, and the like (called plutonic first by 

 Lyell), and other igneous rocks and lavas. But it is a false distinction; for 

 granite is no more of deep-seated origin than other igneous kinds. 



3. The ejections, making dikes and surjicial streams. — The ejected rock 

 may fill a fissure, or but partly fill it. On the other hand, it may flow out of 

 a fissure in a stream over the surface of the country, covering the exposed 

 rocks or soil. The part of the flow within the fissure is a dike, whether 

 there is an outflow or not. Fig. 219, on page 262, represents a dike with 

 a surficial stream. 



(a) Dikes. — Dikes vary in width from an inch or two to 300 feet or 

 more, and in position from vertical to horizontal, and, as already explained, 

 are usually, unless quite small, transversely columnar. The smallest are 

 branches from a larger ; for an inch-thick stream could not flow far between 

 cold rocks. They often have irregularities and interruptions which are due 

 to a faulting of the rocks intersected subsequent to their formation, and 

 others owing to a shifting of the position of the walls of the fissure 

 before it became filled. But, further, there may be, before the filling, a 

 tumbling in of one wall, or the other, of the fissure, especially when the 

 fissure is much inclined and the intersected rock a weak one. 



On the following map, two trap dikes, of the region near New Haven, 

 Conn., are represented (inclosed by dotted lines), which are divided into 

 short parts, owing to the caving in of the overhanging wall. 



The Pine Rock dike consists of four such parts (A,B,CC,D), and Mill 

 Rock of three (AA,BB, to "Peak" and C). The inclination of the dike of 



