BLACK HILLS GEOLOGY. 235 



exerted a powerful influence on the form of the intrusion. In 

 the Algonkian areas, where the schists and slates are tilted on 

 end, the lines of least resistance lie in an approximately vertical 

 direction, and we have a great profusion of dikes, conforming 

 without exception to the strike and dip of the slates. Only 

 when the intruded mass has been large and the force of intru- 

 sion very great do we find irregularities, and even then the gen- 

 eral trend of the masses shows a pronounced parallelism to the 

 lamination of the Algonkian. So soon, however, as the erup- 

 tives reach the Cambrian formation a complete reversal of con- 

 ditions takes place. The lines of least resistance lie now in a 

 horizontal direction, and eruptives on encountering the heavier 

 members of this formation have found it easier to insinuate 

 themselves between the easily cleavable shales and sandstones 

 than to break through the heavy overlying rocks. Therefore 

 we find the predominant type of intrusion in the Cambrian 

 formation to be the intruded sheet. The jointed character of 

 the sandstones and the easy compressibility of the shales be- 

 tween has, however, caused many fractures and faults, resulting 

 in the production of dikes and irregular bodies and modifying 

 the usual horizontal type of intrusion. By such fractures the 

 magmas have been allowed to penetrate all horizons and intro- 

 duce themselves between the shales wherever they have found 

 an unyielding roof to bar their further progress. If the intruded 

 mass has been large, and the force of intrusion great, not only 

 has the rock spread out between the sediments, but it has domed 

 up those which overlay it, producing a laccolite. 



In the Cambrian formation we can distinguish three separate 

 horizons which are most commonly invaded by the eruptives. 



1st. That between the Algonkian and the heavy basal quartz- 

 itic conglomerate which immediately overlies it. 



2d. That underlying the heavy upper quartzite. 



3d. Immediately at the top of the Cambro-Silurian, under 

 massive limestone of the Carboniferous. 



In addition, sheets occur at all the intervening zones, their 

 choice of horizon being influenced by local variations in the thick- 

 ness of the alternating shales and sandstones of the Cambrian. 



