268 J. F. XEWSOM CLASTIC DIKES 



asphalt veins and cla}^, shale, and sand " veins " in coal beds, and clay, 

 sand, and orravel " pipes " : 



1. Dikes of asphalt, clay, gravel, bituminized and unbituminized 

 sands, hard sandstones, and limestone have been described. 



2. The rocks intersected by these dikes include granite, sandstones, 

 sand, shale, cla3^ and limestone. 



3. If the authors are correct in their deductions, clastic dikes have 

 been formed in the following waj^s : 



(a) By injection of material from below along with water, petroleum, 

 or petroleum residues. The injection has been due to hydrostatic press- 

 ure, pressure from overlying beds, pressure from gas, or from combina- 

 tions of these. Injection from below is the commonest method of their 

 formation. 



(h) By injection from above. 



(c) By material dropping into open fissures from above with or with- 

 out the aid of water. 



(d) By material being let down gradually from above, synchronously 

 with the slow formation (by leaching of water) of openings in under- 

 lying calcareous rocks. 



{e) By deposition of sediments in fissures, partially or entirely under 

 the sea. 



4. Coal beds are particularly favorable for the formation of clay and 

 sand intrusions.* 



In closing this discussion, attention is directed to the fact that condi- 

 tions favorable to the formation of clastic dikes b}^ intrusion are pro- 

 duced when any unsolidified sedimentary deposit, be it sand, clay, or 

 calcareous material, is covered by a later deposit of any kind which solid- 

 ifies before the solidification of the underl3dng sediments. Under such 

 conditions the entrapped unsolidified sediments may be forced into joints 

 or fissures in the enclosing hardened rocks, should such fissures be formed. 

 Clastic dikes may also be produced by the pressure of overlying strata 

 squeezing soft unresisting rocks, such as shales, into cracks in either 

 overlying or underlying rocks which have a greater crushing strength 

 than the entrapped beds. 



When these facts are borne in mind, it no longer seems surprising that 

 clastic dikes should occur in great variety, both as to composing mate- 

 rials and surrounding conditions. 



* It is not surprising that beds of vegetable matter undergoing the changes necessary to convert 

 them into coal should become much fissured, neither is it surprising that clay and sand from the 

 strata lying above and below should be squeezed into those fissures, forming the " veins," " dikes," 

 "spars," etc., of the coal miners. The "swelling" or creeping of shale (due to pressure) in the 

 floors of deep coal mine workings is very suggestive of the manner in which sand, clay, and 

 shale dikes in coal beds have been produced. 



