Populate Matrix in C with rand()

I am trying to understand how a Matrix works in C. I have the following code:

#include <unistd.h>
#include <stdlib.h> 
#include <time.h>
#include <stdio.h>

int main(int argc, char* argv[]){

/* random number generator for matrix dimensions */
int xDim, yDim;
srand(time(NULL)); //init. is needed only once
xDim = (rand() % (10000+1) + 50);
yDim = (rand() % (10000+1) + 50);

/* random number generator for matrix contents */
double* myMatr;
myMatr = (double *)malloc(xDim * yDim * sizeof(double));
for(int i=0; i<xDim; i++){
    for(int y=0; y<yDim; y++){
        myMatr[i][y]= (double)rand()/RAND_MAX*100.0;
    }
}
}   

All i get is the error:

 test.c:24:13: error: subscripted value is neither array nor pointer nor vector
myMatr[i][y]= (double)rand()/RAND_MAX*100.0;

Thanks for any help!